The Anonymous Letters of C Forestier - French Letters, #3

Paris, 1825. Isabelle de Tourzin cannot afford to fall in love with a mortal. Not again. Ageless and nearly invulnerable, she spends her days in private grief and plots to end Malbosc, the powerful, secretive man who seduced her only to leech her magic for more than a century. She was rescued before she had the chance to kill him, and he's eluded her ever since. At last, Isabelle has in her hands a magic compass that will point her toward her quarry.
Infuriatingly, a cunning stranger keeps disrupting her search. The interloper claims to be hunting Malbosc as well. Trust is impossible, but the stranger proves to have useful information. Isabelle reluctantly agrees to an alliance. The stranger-a shameless liar who appears to be a man named Forestier-accompanies Isabelle across France. He's charming and unexpectedly kind. It's been a lifetime since she enjoyed anyone's company so much, which makes Forestier an even greater danger than Malbosc.
Forever is too long to live with a broken heart.